In 1913, Dr. Bergius in Germany engaged in the research of producing liquid fuel from coal or coal tar through hydrogenation under high pressure and high temperature, subsequently, he was granted a patent concerning direct coal liquefaction technology, which was the first patent in the field and laid the foundation of direct coal liquefaction. In 1927, the first direct coal liquefaction plant in the world was built in Leuna by a German fuel company (I.G. Farbenindustrie). During World War II, there were altogether 12 such kind of plants built and operated with a total capacity of 423×104 t/year, which supplied ⅔ of the aviation fuel, 50% of the motor fuel and 50% of the tank fuel for the German Army. The direct coal liquefaction process of that time adopted: bubble type liquefaction reactor, filter or centrifuge for solid-liquid separation, iron containing natural ore catalyst. As the recycling solvent separated from the step of filtration or centrifugation contained less reactive asphaltene together with the low activity of the liquefaction catalyst, the operating conditions of liquefaction reaction were very severe, the operating pressure was about 70 MPa and the operating temperature about 480° C.
After World War II, all of the coal liquefaction plants in Germany were shut down. The early 70's oil crisis compelled the developed countries to pay great attention to searching for oil substitutes, thus many new technologies for direct coal liquefaction were studied and developed.
In the early stages of the 1980's, H-COAL process was developed in the USA. In the H-COAL process, a suspended bed reactor with forced circulation was employed, the operating pressure was about 20 MPa and the operating temperature about 455° C. The catalyst used was Ni—Mo or Co—Mo with γ-Al2O3 as carrier, which was the same as the hydrotreating catalyst used in petroleum processing. Recycling solvent was separated by hydrocyclone and vacuum distillation. By virtue of the suspended bed reactor with forced circulation and the hydrotreating catalyst employed in the process, the reaction temperature could be easily controlled and the quality of products stabilized. However, in the coal liquefaction reaction system the hydrotreating catalyst, originally used for petroleum processing, was quickly deactivated, and had to be replaced after a short period of time, which resulted in high cost of the liquid oil products.
The IGOR+ process was developed in the late 1980's in Germany. It employed a bubble type reactor, a vacuum tower to recover the recycle solvent and an on-line fixed bed hydrotreating reactor to hydrogenate both the recycle solvent and the products at different levels. Red mud was used as the catalyst of the process. Since the process employed hydrogenated recycle solvent, coal slurry thus prepared had a stable property and a high coal concentration. Moreover, it could be easily preheated and could exchange heat with gases from the high temperature separator, thus a high heat recovery rate was attained. However, due to the low catalyst activity of the red mud, the operating parameters adopted were still rather severe. The typical operating conditions were as follows: reaction pressure 30 MPa, reaction temperature 470° C. The fixed bed on-line hydrotreating reactor was still at the risk of a short operating cycle due to catalyst deactivation by coking. In addition, the precipitation of calcium salts in the bubble type reactor was unavoidable, if the calcium content of the coal feed was high.
In the late 1990's, the NEDOL process was developed in Japan. In the NEDOL process, a bubble type reactor was also used, the recycle solvent was prepared by vacuum distillation and hydrotreated in an off-line fixed bed hydrogenation reactor, and ultrafine pyrite (0.7μ) was used as liquefaction catalyst. In the process, all recycling hydrogen donor solvent was hydrogenated, thus the coal slurry properties were stable and it could be prepared with a high coal concentration. Moreover, the coal slurry could be easily preheated and could exchange heat with gases from the high temperature separator. Therefore a high heat recovery rate was attained. Additionally, the operation conditions of the process were relatively mild, for example, the typical operating conditions were as follows: reaction pressure 17 MPa, reaction temperature 450° C. However, owing to the hardness of the pyrite ore, it was quite difficult to pulverize into super-fine powder, thus the cost of catalyst preparation was high. For the bubble type reactor, due to its high gas holdup factor, the reactor volume utilization rate was low. Besides, due to a low liquid velocity in the reactor, precipitation of organic minerals might occur, and for the fixed bed hydrotreating reactor employed in the process the risk of short operating cycle still existed.